Low-frequency magnetic reluctance marine seismic source

ABSTRACT

This disclosure is related to marine seismic sources, for example marine seismic sources known in the art as benders. Some embodiments of this disclosure use magnetic reluctance forces to produce seismic energy. For example, pole pieces may be attached to one or more plates of a marine seismic source, and a wire coil may induce an attractive force between the pole pieces to cause deformation of the plates to produce seismic energy. Such marine seismic sources may be components of a marine seismic survey system, and may be used in a method of marine seismic surveying. Methods of making marine seismic sources are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 14/536,190filed Nov. 7, 2014 titled, “Low-Frequency Magnetic Reluctance MarineSeismic Source”, which claims the benefit of Provisional PatentApplication No. 61/920,141, filed Dec. 23, 2013. Both applications areincorporated by reference herein as if reproduced in full below.

BACKGROUND

In the oil and gas exploration industry, marine geophysical prospectingis commonly used in the search for hydrocarbon-bearing subterraneanformations. Marine geophysical prospecting techniques may yieldknowledge of the subsurface structure of the Earth, which is useful forfinding and extracting hydrocarbon deposits such as oil and natural gas.Seismic surveying is one of the well-known techniques of marinegeophysical prospecting.

In some instances of seismic surveying conducted in a marine environment(which may include saltwater, freshwater, and/or brackish waterenvironments), one or more marine seismic sources are typicallyconfigured to be submerged and towed by a vessel. The vessel istypically also configured to tow one or more laterally spaced streamersthrough the water. At selected times, control equipment may cause theone or more marine seismic sources to actuate. Seismic signals may thenbe received by sensors disposed along the streamers. Data collectedduring such a seismic survey may be analyzed to assist identification ofhydrocarbon-bearing geological structures, and thus determine wheredeposits of oil and natural gas may be located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIGS. 2A-2B illustrate a disassembled view of an embodiment of a marineseismic source according to this disclosure.

FIGS. 3A-3B illustrate a disassembled view of another embodiment of amarine seismic source according to this disclosure.

FIG. 4 illustrates a cross-sectional view of an embodiment of a marineseismic source according to this disclosure.

FIG. 5 illustrates a cross-sectional view of an embodiment of a portionof a marine seismic source according to this disclosure.

FIG. 6 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIG. 7 illustrates an embodiment of a marine seismic source according tothis disclosure.

FIGS. 8-9 illustrate graphs related to some embodiments of thisdisclosure.

FIG. 10 illustrates an embodiment of a marine seismic source accordingto this disclosure.

FIG. 11 illustrates an embodiment of a marine seismic source accordingto this disclosure.

FIGS. 12-13 illustrate embodiments of methods according to thisdisclosure.

DETAILED DESCRIPTION

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Various devices, units, circuits, or other components may be describedor claimed as “configured to,” “usable to,” or “operable to” perform atask or tasks. In such contexts, “configured to,” “usable to,” and“operable to” are each used to connote structure by indicating that thedevices/units/circuits/components include structure that performs thetask or tasks during operation. As such, thedevice/unit/circuit/component can be said to be configured to, usableto, or usable to perform the task even when the specifieddevice/unit/circuit/component is not currently operational (e.g., is noton or in operation). The devices/units/circuits/components used with the“configured to,” “usable to,” or “operable to” language may includeelectronic hardware—for example, circuits, memory storing programinstructions executable to implement the operation, etc.—mechanicaldevices, or other types of structure. Reciting that adevice/unit/circuit/component is “configured to,” “usable to,” or“operable to” perform one or more tasks is expressly intended not toinvoke 35 U.S.C. §112(f), for that device/unit/circuit/component.

In some embodiments, various items of information relating to seismicsurveying may be embodied in a geophysical data product. A “geophysicaldata product” may be stored on a computer-readable, non-transitorymedium and may embody geophysical data (such as raw streamer data,processed streamer data, two- or three-dimensional maps based onstreamer data, etc.). Some non-limiting examples of computer-readablemedia may include hard drives, CDs, DVDs, print-outs, etc. In someembodiments, raw analog data from streamers may be stored as ageophysical data product. In other instances, the data may first bedigitized and/or conditioned prior to being stored as the geophysicaldata product. In yet other instances, the data may be fully processedinto a two- or three-dimensional map of the various geophysicalstructures before being stored in the geophysical data product. Thegeophysical data product may be produced offshore (e.g., by equipment ona vessel) or onshore (e.g., at a facility on land) either within theUnited States or in another country. If the geophysical data product isproduced offshore or in another country, it may be imported onshore to afacility in the United States. Once onshore in the United States,geophysical analysis may be performed on the geophysical data product.

In some instances of a typical marine seismic survey, one or more marineseismic sources may be used to generate seismic energy. The seismicenergy may travel downward through the water and through the formationsunderlying the water bottom. Impedance boundaries within the subseaformations may reflect (at least in part) the seismic energy thattravels through such formations. The reflected seismic energy may thentravel upwards. Geophysical sensors (e.g., hydrophones, geophones,accelerometers, etc.) may capture such reflected seismic energy. Thesegeophysical sensors may convert the captured seismic energy intosignals, such as optical or electrical signals. The electrical oroptical signals may then be interpreted to provide information as to thecomposition and/or structure of the various subsurface formations underthe water bottom. Such information may be used, for example, todetermine the possibility that such formations may contain mineraldeposits including hydrocarbons.

One type of marine seismic source that may be used in seismic surveyingoperations is a flexural disc projector, commonly known as a “bender.” Atypical bender may employ one or more piezoelectric elements, such thatthe mechanical vibration of the bender is driven by piezoelectricdistortion based on electrical energy applied to the piezoelectricelement. For example, when electrical energy (e.g., a voltage) isapplied to a piezoelectric material, a volume or length of thepiezoelectric element may increase or decrease in response. Thisphenomenon is generally termed the “inverse piezoelectric effect,” andit may be exploited in order to produce seismic energy. For example, apiezoelectric element may be attached (e.g., glued) to a portion of amarine seismic source, such that the contractions and/or expansions ofthe piezoelectric element may cause deformations in the portion, and thedeformations may cause seismic energy to be transmitted through thewater.

However, although common, typical piezoelectric benders may be subjectto various disadvantages in some cases. For example, they are oftenlabor-intensive and expensive to produce. Further, large piezoelectricsmay be especially difficult to produce, which can effectively limit thesize of piezoelectric benders. Piezoelectric elements may also be proneto cracking, as well as being sensitive to pressure variations.Accordingly, as described more fully below, some embodiments of thisdisclosure include marine seismic sources that use the magneticreluctance force (e.g., the force experienced due to a material'smagnetic reluctance in the presence of magnetic flux) to produce seismicenergy. This new technique may ameliorate some of the difficulties thathave arisen with piezoelectric marine seismic sources. For example, arelatively large force or pressure between the plates of a bender (andthus relatively high-amplitude seismic energy) may be achieved with arelatively low-mass bender. Various other advantages will also becomeclear with the benefit of this disclosure.

When seismic energy travels through water and subsea formations, higherfrequency seismic energy may typically be attenuated more rapidly thanlower frequency waves. Accordingly, seismic energy at lower frequencies(for example, infrasound) may typically be transmitted over longerdistances through water and subsea formations than higher frequencyseismic energy. A marine seismic source that produces seismic energy atlower frequencies may thus have utility in marine seismic surveys,particularly those conducted in increased water depths.

The design parameters for a marine seismic source may include, amongothers, seismic energy output within a low frequency range (e.g.,between 2 Hz and 20 Hz, or between 2 Hz and 10 Hz), water depth, weight,size and cost. For example, it may be advantageous for a marine seismicsource to be capable of supplying broadband low-frequency energy, e.g.,such that the frequency spectrum may be changed as desired according tothe geophysical properties in a survey. As modern marine seismic surveyscontinue to explore waters at increased depths, there is a continuingneed for a low cost (for example, both in terms of manufacturing costand operating cost) marine seismic source that produces high seismicenergy at lower frequencies, particularly at frequencies in the range ofapproximately 2 Hz to 10 Hz. Some embodiments of this disclosure areparticularly applicable in this frequency range, although otherembodiments are operable in larger ranges, such as 2 Hz to 20 kHz.

In the context of a low-frequency source (which generally refers to asource outputting a frequency in the range of 2-10 Hz), it may beadvantageous to tow the source at a depth of approximately λ/4, where λis the wavelength of the seismic energy being output by thelow-frequency source. For example, since seawater has a sound velocityof approximately 1,500 meters/second, λ/4 for a 5 Hz source would beapproximately 75 meters. For a frequency in the range of 2-10 Hz, λ/4would be between approximately 35 and 190 meters. For a frequency in therange of 2-20 Hz, λ/4 would be between approximately 15 and 190 meters.

Turning now to FIG. 1, an exemplary illustration of marine seismicsource 100 is shown. As shown, marine seismic source 100 may be in theform of an electromechanical device such as a bender. It should be notedthat FIG. 1 (as well as the rest of the Figures) may not necessarily bedrawn to scale. In some embodiments, a marine seismic source accordingto this disclosure may have a larger or a smaller height relative to itsdiameter.

Marine seismic source 100 in this embodiment is generally cylindrical,and it is arranged about axis 102. Marine seismic source 100 includestop plate 104, as well as a corresponding bottom plate 150 (not showndue to the perspective of FIG. 1). These plates are held in place viafasteners 106 (such as bolts), which connect them to hoop 108. Hoop 108extends around the circumference of marine seismic source 100 andprovides a desired separation between top plate 104 and bottom plate150. Hoop 108 may also provide a fixed contact against which top plate104 and bottom plate 150 can flex inwardly and/or outwardly.

One of ordinary skill in the art with the benefit of this disclosurewill understand that the references to “top” and “bottom” plate are notintended to indicate that the top plate must necessarily be above thebottom plate. These designations are instead intended only to simplifythe descriptions of the drawings in this disclosure. In embodiments ofmarine seismic sources according to this disclosure, the “top” and“bottom” plates may be in any desired physical orientation relative toone another.

An interior cavity may be formed between top plate 104 and bottom plate150, and this interior cavity may be configured to contain a volume ofgas (such as air, nitrogen, or any other suitable substance). In someembodiments, the volume of gas within the interior cavity may bepressurized. In marine applications, pressurizing and maintaining thevolume of gas within the interior cavity at an ambient hydrostaticpressure at a selected operating water depth may protect marine seismicsource 100 from collapsing from the ambient hydrostatic pressure.

Plates 104 and 150 may each be of a circular shape, an elliptical shape,or some other shape. Plates 104 and 150 may be made from a material suchas metal (e.g., aluminum, steel, etc.). Plates 104 and 150 may be madefrom the same material in one embodiment. Yet in another embodiment,they may be made from different materials.

Not shown in FIG. 1, various types of circuitry and components may beused to produce seismic energy by causing top plate 104 and/or bottomplate 150 to move. As noted above, one possibility is the use of one ormore piezoelectric elements (e.g., piezoelectric ceramics glued to theinterior or exterior surfaces of the plates). In accordance with thisdisclosure, however, pole pieces, wire coils, etc. may in someembodiments also be used to cause marine seismic source 100 to produceseismic energy.

Turning now to FIGS. 2A and 2B, views are shown respectively of topplate 204 and bottom plate 250 of marine seismic source 200. A hoopoperable to connect these two plates (e.g., corresponding to hoop 108 inFIG. 1) is not shown, and axis 202 is shown as passing through both topplate 204 and bottom plate 250. When assembled, top plate 204 and bottomplate 250 would be aligned along axis 202.

Top plate 204 in this embodiment includes pole piece 210 attachedthereto. Pole piece 210 may typically be made of a ferromagneticmaterial. In this embodiment, pole piece 210 is shaped as a diskarranged concentrically with top plate 204. Pole piece 210 may furtherinclude groove 212, which will be discussed in more detail below. Insome embodiments, pole piece 210 may be attached to the bottom surfaceof top plate 204. In other embodiments, pole piece 210 may extend atleast partially through the interior of top plate 204, beingmagnetically accessible to the bottom surface of top plate 204. Polepiece 210 may be any suitable type of pole piece, such as iron, steel,cobalt, various alloys, and/or any other material having suitablemagnetic properties.

Bottom plate 250 is shown with pole piece 260 positioned such that itwould be adjacent to pole piece 210 in the assembled device. In someembodiments, pole piece 260 may be attached to the top surface of bottomplate 250. In other embodiments, pole piece 260 may extend at leastpartially through the interior of bottom plate 250, being magneticallyaccessible to the top surface of bottom plate 250. Similar to groove 212in pole piece 210, pole piece 260 may include groove 262. Grooves 212and 262 are arranged in this embodiment such that they would be adjacentto one another in the assembled device. Wire coil 270 in this embodimentis disposed within groove 262 of pole piece 260. One of ordinary skillin the art will understand that in some embodiments, wire coil 270 maybe of sufficient diameter that in the assembled device it may extend atleast partially into groove 212. In other embodiments, wire coil 270 maybe entirely contained within groove 262, while in yet other embodiments,wire coil 270 may be entirely contained within groove 212. In some suchcases, one or the other of grooves 212 and 262 may even be omittedentirely.

Accordingly, in some cases top plate 204 and bottom plate 250 may besubstantially identical to one another. In some cases, however, they maybe different in various ways. In the embodiment shown, for example,grooves 212 and 262 may line up with one another in the assembled deviceto form a channel through which wire coil 270 may run. Wire coil 270 mayconsist of a single loop of wire or multiple loops of wire, asappropriate. Various electronic components, not shown in FIGS. 2A and2B, may be used to supply electrical energy to wire coil 270.

Pole pieces 210 and 260 and/or wire coil 270 may in some embodiments beattached by an adhesive, such as epoxy, or they may be bolted in place,or they may be attached via any other suitable means. There is nolimitation as to how such components may be attached to plates 204 and250. As used herein, the words “attach,” or “attached” (and otherderivations thereof) should be understood to mean a connection betweencomponents, whether direct or indirect. Further, in some embodiments,wire coil 270 may simply be placed in a groove without being attached.

When electrical energy is supplied to wire coil 270, this may cause topplate 204 and/or bottom plate 250 may bend, flex, or otherwise bedistorted. For example, a force may arise between pole pieces 210 and260 due to an induced magnetic pressure. That is, the passage ofelectrical current through wire coil 270 may cause the system to behaveas an electromagnet with a split core, with portions (e.g., pole pieces210 and 260) of the core attached to each plate.

A current in wire coil 270 may thus induce a magnetic field that causespole pieces 210 and 260 to attract one another, and thus may cause topplate 204 and bottom plate 250 to be likewise attracted to one another.Through suitable regulation of the supplied voltage and/or current, thisattractive force may result in vibration and seismic energy output. Inthis embodiment, the plates may bend, flex, or otherwise be distortedaxially along axis 202. In some embodiments, the distortions may beaxially symmetric or symmetric with respect to axis 202. According toone embodiment, a typical driving current may be approximately 5 A/mm²,and the driving current may be AC or DC in different embodiments.According to some embodiments, a typical flux density of approximately 1Tesla may be employed and may generate an attractive magnetic pressureof approximately 4 Bar between the plates of a marine seismic source. Ingeneral, various embodiments of this disclosure may typically provideflux densities in the range of approximately 0.5-2.2 Tesla andattractive magnetic pressure in the range of approximately 1-20 Bar.

According to some embodiments, it may be advantageous for groove 212 tobe disposed in pole piece 210 (and correspondingly groove 262 in polepiece 260) such that the area of the pole piece that is inside thegroove is equal (or approximately equal) to the area of the pole piecethat is outside the groove. For example, the areas may in someembodiments advantageously be within 1%, within 2%, within 3%, within4%, within 5%, within 10% of one another, etc. One possible reason forsuch an arrangement is that when the current in the wire coil is large,this arrangement may “spread out” the flux in a way that allows for agreater total flux without saturating the pole pieces. Accordingly,greater forces/pressures may be achieved.

Turning now to FIGS. 3A and 3B, marine seismic source 300 is shown.Marine seismic source 300 is broadly similar to marine seismic source200, but pole pieces 210 and 260 have each been replaced by a pluralityof pole pieces 310 and 360, respectively. (In these Figures and others,similar reference numerals are given to corresponding components—e.g.,top plate 204 corresponds to top plate 304, etc.) Similar to the groovesin marine seismic source 200, grooves 312 and 362 are disposed alongthese pluralities of pole pieces, and wire coil 370 runs through one orboth of such grooves.

FIGS. 3A and 3B thus illustrate that devices according to thisdisclosure need not be limited to a single pole piece on each plate, butthat any desired number and configuration of pole pieces may be used.For example, a larger number of smaller pole pieces may be more costeffective in some instances. In general, one or more pole pieces on eachplate of a marine seismic source may be used.

As discussed above, embodiments of this disclosure rely on the magneticreluctance force (also known as the magnetomotive force) to produceseismic energy. Without wishing to be limited by theory, in someembodiments the magnetic reluctance force may be briefly described asfollows. Magnetic reluctance is an attractive force that tends tominimize air gaps in magnetic circuits. The magnitude of this force maydepend on various physical properties of the components included inembodiments of this disclosure—e.g., area and magnetic reluctance ofpole pieces, number of turns in wire coil, distance between pole pieces,etc.

The force provided by magnetic reluctance in some embodiments of marineseismic sources according to this disclosure may be given by thefollowing equation:

$F = {\frac{\varnothing^{2}}{\mu_{0}A_{gap}} = {\left\{ {\varnothing = \frac{IN}{R}} \right\} = {\frac{I^{2}N^{2}}{R^{2}\mu_{0}A_{gap}} = {\left\{ {R = \frac{x}{\mu_{0}A_{gap}}} \right\} = \frac{I^{2}N^{2}\mu_{0}A_{gap}}{x^{2}}}}}}$whereF=Attraction force [N]ø=Total Magnetic Flux [Wb]μ₀=Permeability of vacuum

$\left\lbrack \frac{H}{m} \right\rbrack$A_(gap)=Cross sectional area of pole piece [m²]I=Current in coil [A]N=Number of turns in coil [-]R=Ideal Reluctance of air gap

$\left\lbrack \frac{A}{Vs} \right\rbrack$x=Distance between pole pieces [m]

Accordingly, as described in more detail below, the attractive forcebetween the plates of a marine seismic source according to thisdisclosure may be nonlinear, being proportional to the inverse square ofthe separation of the pole pieces.

Turning now to the issue of the amount of seismic energy produced, it isnoted that a maximum sound pressure level of a marine seismic source maytypically occur at or near a mechanical resonance frequency of themarine seismic source. Broadly speaking, sound pressure level is adifference, in a given medium, between a pressure associated with theseismic energy and an average local pressure. The square of thisdifference may be averaged over time and/or space, and a square root ofthis average may provide a root-mean-square (RMS) pressure value orP_(rms). Sound pressure level is a logarithmic measure indicating aratio of a given P_(rms) relative to a reference sound pressure orP_(ref). Sound pressure level is typically measured in decibels (dB). Inmarine applications, a reference pressure P_(ref) is usually 1micropascal (1 μPa). In mathematical terms, sound pressure level may becalculated by the equation below:Sound Pressure Level (dB)=20 log(P _(rms) /P _(ref))

One of ordinary skill in the art with the benefit of this disclosurewill understand that the diameter, thickness, and material constructionof top plate 204 and bottom plate 250 may advantageously be designed inorder to achieve desirable operational characteristics (e.g., aresonance frequency in a selected range, such as 2-20 Hz). This mayprovide a sufficiently large sound pressure level in the frequency rangeof interest. According to one embodiment, the top plate and bottomplates may be approximately 450-700 mm in diameter. According to oneembodiment, they may also be approximately 3-5 mm in thickness. Anysuitable material may be used, such as aluminum, steel, other metals,alloys, composites, etc.

Turning now to FIG. 4, a cross-sectional view of an embodiment of marineseismic source 400 is shown. Marine seismic source 400 is arranged aboutaxis 402, and it includes top plate 404 and bottom plate 450 separatedby hoop 408. In this illustration, top plate 404 has attached thereto asingle pole piece 410, and bottom plate 450 has attached thereto asingle pole piece 460. Grooves 412 and 462 are respectively disposedwithin pole pieces 410 and 460, and wire coil 470 is disposed within oneor both of those grooves. In the embodiment of FIG. 4, wire coil 470 isshown as “floating” between pole pieces 410 and 460; in otherembodiments, however, it may be attached to one or both of pole pieces410 and 460. The right side of wire coil 470 is shown in this embodimentas going “into the page,” and the left side is shown is coming “out ofthe page”; this is to indicate the axial symmetry of the current thatmay be passed through wire coil 470.

One of ordinary skill in the art with the benefit of this disclosurewill understand that the Figures are not necessarily drawn to scale, andwill further recognize that the spacing between pole pieces 410 and 460may advantageously be selected in order to produce the desired forces.Further, pole pieces 410 and 460 may in some embodiments advantageouslybe designed such that they do not come into physical contact with oneanother during normal operation of marine seismic source 400.

Turning now to FIG. 5, a cross-sectional detail view of a portion of amarine seismic source is shown. This marine seismic source is arrangedabout axis 502 and includes wire coil 570 and pole pieces 510 and 560.(Respective top and bottom plates are not shown in this illustration forthe sake of simplicity.) When an electrical current is passed throughwire coil 570, a magnetic field may be induced as shown at magneticfield lines 580. This magnetic field may provide an attractive forcebetween pole pieces 510 and 560 and the respective top and bottom plates(not shown).

In FIG. 5, wire coil 570 is shown as a single wire of circularcross-section. One of ordinary skill in the art with the benefit of thisdisclosure will understand that in other embodiments, a plurality ofwires may be used. For example, in some embodiments, a larger number ofsmaller wires may be used, substantially filing one or both of thegrooves in the pole pieces.

Due (for example) to differences in mass between pole pieces and wirecoils, it may sometimes be the case that the combined mass of one plate(that is, the mass of the plate plus the mass of any pole pieces, wirecoils, and other components attached thereto) is larger than thecombined mass of the opposite plate. In some cases, however, it may beadvantageous for the plates of a marine seismic source according to thisdisclosure to have equal mass (or approximately equal mass, such aswithin 1% of one another, within 2% of one another, within 3% of oneanother, within 4% of one another, within 5% of one another, within 10%of one another, etc.). Equalizing the mass of the two plates may haveadvantageous effects on the sound pressure level, the resonancefrequency, the frequency spectrum, and/or other characteristics of someembodiments of marine seismic sources according to this disclosure.

Although mass equalization may in some instances be achieved by simplyadding mass to the lighter of the plates, it may also be achieved insome cases by arranging a marine seismic source according to thisdisclosure such that the plates themselves have approximately equalmass, and further such that the other components attached to each platealso have approximately equal mass. For example, if the wire coil isattached to the bottom plate, then the top plate (or pole piece attachedthereto) may be designed with a somewhat higher mass to account for themass of the wire coil.

As noted above, the magnetic reluctance forces between the pole piecesaccording to this disclosure are typically attractive forces.Accordingly, in some embodiments, it may additionally be advantageous toprovide a repulsive force that may partially counteract suchforces—e.g., to provide a force that will resist the attractive magneticreluctance force. For example, such a repulsive force may act on thepole pieces, and/or on the top and bottom plates themselves. By varyingthe current provided to the wire coil in the presence of such arepulsive force, an oscillating motion may be established to provideseismic energy. There are many ways of providing such a repulsive force,and the following discussion provides a few examples. One of ordinaryskill in the art with the benefit of this disclosure will of courseenvision many possible variations on ways of providing a repulsive forceto resist the attractive magnetic reluctance force. These may in variousembodiments be used in isolation or in combination with one another.

With momentary reference back to FIG. 4, one example of a way ofproviding resistance to the attractive magnetic reluctance force is touse the stiffness of various components to provide an effective springforce opposing the magnetic reluctance force. For example, the thicknessand arrangement of top and bottom plates 404 and 450, as well as hoop408 may be selected in order to produce a desired force and/or a desiredequilibrium separation distance between the pole pieces. The thicknessand arrangement of pole pieces 410 and 460 may also in some embodimentscontribute to this force, and thus their design may also be determinedwith regard to the desired force. According to some embodiments, theequilibrium separation of the pole pieces may be selected such that theyare sufficiently close to allow the magnetic reluctance force todominate the spring force at acceptable current levels, but sufficientlyfar that they do not contact one another during normal operation. Forexample, in some embodiments, an equilibrium separation of approximately0.5-5 mm may be used.

Another way of providing a repulsive force in some embodiments is topressurize the interior of marine seismic source 400 (e.g., with air,nitrogen, or some other suitable gas). For example, a pressure in therange of 1-12 Bar may provide a suitable force or pressure between topplate 404 and bottom plate 450 of marine seismic source 400.

Turning now to FIG. 6, marine seismic source 600 is shown. Marineseismic source 600 is broadly similar to marine seismic source 400, butwith additional structures to provide a repulsive force between topplate 604 and bottom plate 650. Pole piece 660 includes cylinder 656,and pole piece 610 includes gas piston 616, which fit together inoperation to trap air (or other gas). As the plates move together, theair in cylinder 656 becomes increasingly compressed, providing arepulsive force. One of ordinary skill in the art with the benefit ofthis disclosure will understand that the size of cylinder 656 and gaspiston 616 may be selected to provide the desired amount of force. Thisrepulsive force may be used to prevent the pole pieces from contactingeach other in operation, and they may also be used to compensatepartially for the non-linear properties of the system (described in moredetail below with reference to FIG. 9).

Turning now to FIG. 7, marine seismic source 700 is shown. Marineseismic source 700 is broadly similar to marine seismic source 600, butwith a different way of providing a repulsive force between top plate704 and bottom plate 750. Pole pieces 710 and 760 include magnets 720and 758, respectively. Magnets 720 and 758 may be arranged with oppositepolarity in order to provide a repulsive force, e.g., for the purposesdescribed above. In some embodiments, however, they may be arranged withthe same polarity to provide an attractive force. This may be used, forexample, to identify a desired value for the equilibrium spacing betweenpole pieces 710 and 760.

Turning now to FIG. 8, a graph is shown that illustrates the forcesbetween the plates of an embodiment of a marine seismic source accordingto this disclosure. Magnetic reluctance force 802 (which is attractive)is shown, as well as spring force 804 (which is repulsive). Thisembodiment relies solely on the spring force to provide repulsive forcebetween the plates, as described above. As shown, the equilibriumseparation between the pole pieces (the position at which spring force804 is zero, and so the resting position at zero current) is about 5 mm.In this example, the effective spring constant is k=1 mm/kN, but one ofordinary skill in the art with the benefit of this disclosure willunderstand that any desired spring constant may be used to achieve thedesired system characteristics.

In this example, the other mechanical properties of the device aregenerally as follows:

TABLE 1 Pole piece radius 101 mm Pole piece height 40 mm Coil innerradius 55 mm Coil outer radius 85 mm Coil height 20 mm Total weight 10kg

FIG. 8 shows that the magnetic reluctance force may be nonlinear as afunction of the distance between the pole pieces. That is, as the polepieces get closer together, the strength of the interaction may increasevery quickly. As discussed above, in some embodiments, the strength maybe inversely proportional to the second power of the separation.

FIG. 8 shows the magnetic reluctance interaction at a constant currentof 5 A/mm², but one of ordinary skill in the art with the benefit ofthis disclosure will recognize that this need not be the case. It may beadvantageous to employ a control system that (for example) reduces thecurrent level as the pole pieces approach one another in order tocounteract the nonlinearity of the interaction. Control systems may alsobe utilized to account for the magnetic hysteresis of the pole pieces(e.g., the non-zero time required for magnetic flux to change inresponse to a change in current). Some embodiments are described in moredetail below.

The system illustrated in FIG. 8 has been designed such that the amountof available attractive magnetic reluctance force is greater than therepulsive spring force. This may be advantageous, because it means thatthe entire range of separations below the equilibrium separation of 5 mmis accessible to this marine seismic source. If the spring force weregreater than the available magnetic reluctance force, then there may beregions that could not be reached.

Turning now to FIG. 9, an additional graph is shown that illustrates theforces between the plates of a different embodiment of a marine seismicsource according to this disclosure. This embodiment relies on both thespring force and a gas piston (as shown in FIG. 6) to provide arepulsive force between the plates. Magnetic reluctance force 902 (whichis attractive) is shown, as well as spring force 904 and gas pistonforce 906 (which are repulsive). In this example, the equilibriumseparation between the pole pieces is about 1.5 mm.

As shown in FIG. 9, the nonlinear repulsive force of the gas piston mayadvantageously be combined with the nonlinear attractive magneticreluctance force. That is, by selecting an appropriately sized gaspiston, the rapid increase of the attractive force as the pole piecesapproach one another may (at least partially) be counterbalanced by therapid increase of the repulsive force as the gas is pressurized in thegas piston.

In order to compensate for the nonlinearities of the electromagnet, insome embodiments, a marine seismic source may be designed such that itsinherent spring constant increases with deflection. Such progressivespring characteristics may be achieved, for example, by shaping themarine seismic source and choosing the material properties of thebenders for that purpose.

According to some embodiments, various types of control systems may alsobe used to account for the nonlinearities and/or magnetic hysteresiseffects that may arise in marine seismic sources according to thisdisclosure. For example, Iterative Learning Control (ILC) controllersare known in the art to be useful in the context of reluctanceactuators. Various other types of controllers are also known in the artfor dealing with nonlinearities, hysteresis effects, etc. One ofordinary skill in the art with the benefit of this disclosure willunderstand that any of various types of control systems mayadvantageously be employed.

Turning now to FIG. 10, an embodiment is illustrated in which an arrayof several individual marine seismic sources 1020 may be arranged insidea housing into stack assembly 1000. Although the individual marineseismic sources 1020 are capable of providing seismic energy, it may beadvantageous to combine them in some embodiments into a stack such asstack assembly 1000. Such an arrangement may in some instances be usedto increase the total sound pressure level achievable, relative to whatmay be achievable via a single marine seismic source 1020. In oneembodiment, stack assembly 1000 may include marine seismic sources 1020in a series configuration. In other embodiments, parallel configurationsare possible, as well as embodiments that incorporate both series andparallel components. Stack assembly 1000 may include top plate 1004 andbottom plate 1050 to which marine seismic sources 1020 may be secured.

FIG. 11 illustrates yet another embodiment of a stack assembly in whichstack assembly 1100 additionally includes boot assembly 1102. Bootassembly 1102 may enclose and/or be disposed around the individualmarine seismic sources shown in FIG. 10, for example. In one particularembodiment, boot assembly 1102 may include a liquid (in some cases, anelectrically insulating material such as an electrically insulating oilmay be used) in which the individual marine seismic sources may beimmersed or at least partially disposed. This liquid may serve as anadditional protective layer for the individual marine seismic sources,and it may also provide a medium through which seismic energy istransmitted.

Turning now to FIGS. 12 and 13, flow diagrams are presented inaccordance with aspects of this disclosure. It should be understood thatin some embodiments, fewer than all steps of a particular process flowmay be performed in accordance with this disclosure. Further, it shouldbe understood that in some embodiments, steps may be performed in adifferent order.

FIG. 12 is a flow diagram illustrating an embodiment of a method 1200 ofoperating an apparatus according to this disclosure. Flow begins atblock 1210, in which a marine seismic source is actuated to produceseismic energy. For example, the marine seismic source may be a singlesource (e.g., an individual bender), or in some embodiments it may be anarray of such single sources.

The marine seismic source includes (at least) a first plate having afirst set of one or more pole pieces attached thereto, a second platehaving a second set of one or more pole pieces attached thereto, and awire coil disposed between the first and second plates. For example, asshown in some examples above, the wire coil may be disposed in one ormore grooves in the pole pieces.

The plates may be similar to the various top plates and bottom platesdescribed above. If the marine seismic source includes an array ofsingle sources, then it is contemplated that more than one source in thearray of sources may include first and second plates, and correspondingsets of one or more pole pieces. For example, each source in the arraymay be substantially similar to the others in the array. Variousexamples of suitable configurations have been described above. Flowproceeds to block 1220.

At block 1220, the seismic energy is detected. This may be accomplishedvia any of various types of sensors, such as hydrophones, geophones,accelerometers, etc. The sensors may be located on streamers towed by asurvey vessel, on ocean bottom cables, ocean bottom nodes, or otherwisedisposed in the body of water or subsurface formation. Further, theseismic energy may in some embodiments be detected after it hasinteracted with various subsea formations. Flow ends at block 1220.

FIG. 13 is a flow diagram illustrating an embodiment of a method 1300for making an apparatus in accordance with this disclosure. Flow beginsat block 1310, in which a first set of one or more pole pieces isattached to a first plate. The first set may generally include one ormore pole pieces. Flow proceeds to block 1320.

At block 1320, a second set of one or more pole pieces is attached to asecond plate. The second set may also generally include one or more polepieces. Flow proceeds to block 1330.

At block 1330, a wire coil is attached to a groove in one or more of thepole pieces. For example the wire coil may be disposed entirely in agroove in the first set, entirely in a groove in the second set, orpartially in a groove in the first set and partially in a groove in thesecond set. Flow proceeds to block 1340.

At block 1340, the first plate and the second plate are attached to ahoop. The hoop may in some instances be an annular element extendingaround a circumference of the first and second plates. The plates may beattached to the hoop via, for example, a plurality of bolts. Flow endsat block 1330.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A method comprising: actuating a marine seismicsource to produce seismic energy, the actuating by: passing electriccurrent through a wire coil disposed in a first groove defined in afirst pole piece, the first pole piece coupled to a first plate thatdefines a first central axis and a first outer edge; attracting a secondpole piece toward the first pole piece, the second pole piece coupled toa second plate that defines a second central axis and a second outeredge, the first central axis coaxial with the second central axis, andthe first plate parallel to the second plate; bending the first andsecond plates toward each other along the first and second central axis;and maintaining separation of the outer edges of the first and secondplates by way of a hoop disposed between the first and second plates atthe outer edges; and detecting the seismic energy.
 2. The method ofclaim 1, further comprising towing the marine seismic source behind asurvey vessel in a body of water.
 3. The method of claim 2, wherein themarine seismic source is towed at a depth of between 35 and 190 meters.4. The method of claim 1, further comprising recording the detectedenergy on a tangible, non-volatile computer-readable medium.
 5. Themethod of claim 1, wherein the actuating includes actuating a pluralityof marine seismic sources.
 6. The method of claim 5, wherein theplurality of marine seismic sources are arranged in a seriesconfiguration.
 7. The method of claim 1, wherein passing electriccurrent through the wire coil further comprises passing electric currentthrough the wire coil disposed in the first groove of the first polepiece being a ferromagnetic pole piece.
 8. The method of claim 1,wherein actuating the marine seismic source to produce seismic energyfurther comprises producing seismic energy within a frequency range of 2Hz to 20 Hz at a water depth in a range of 15 to 190 meters.
 9. Themethod of claim 1, wherein actuating the marine seismic source toproduce seismic energy further comprises producing seismic energy withina frequency range of 2 Hz to 10 Hz at a water depth in a range of 35 to190 meters.
 10. The method of claim 1, wherein bending the first andsecond plates toward each other further comprises bending with acombined mass of the first plate and the first pole piece within 5% of acombined mass of the second plate and the second pole piece.
 11. Themethod of claim 1, further comprising: ceasing the flow the electriccurrent through the wire coil; separating the first and second platesalong the first and second central axis such that the first and secondpole pieces have a separation in the range of 0.5 to 5 mm, theseparating by stiffness of the first and second plates.
 12. The methodof claim 1, further comprising: repelling a second pole piece away fromthe first pole piece; bending the first and second plates away from eachother along the first and second central axis; and maintainingseparation of the outer edges of the first and second plates by way ofthe hoop.
 13. The method of claim 12, further comprising repelling thefirst and second plates by way of a plurality of permanent magnetscoupled to the first and second plates.
 14. The method of claim 1wherein passing electric current further comprises passing electriccurrent through the wire coil disposed in the first groove defined in afirst pole piece coupled to the first plate, the first plate iscircular.
 15. An apparatus, comprising: a marine seismic source thatincludes: a first plate that defines a first central axis and a firstouter edge; a second plate that defines a second central axis and asecond outer edge, the first central axis is coaxial with the secondcentral axis, and the first plate is parallel to the second plate; ahoop disposed between the first and second plates at the outer edges ofthe first and second plates; an interior cavity defined between thefirst plate, the second plate, and the hoop; a first pole piece coupledto the first plate, the first pole piece within the interior cavity; afirst groove defined in the first pole piece; a second pole piececoupled to the second plate, the second pole piece within the interiorcavity; a wire coil disposed in the first groove between the first andsecond plates such that the first and second pole pieces are configuredto attract one another when an electric current is passed through thewire coil such that the first and second plates bend toward each otheralong the coaxial first and second central axis.
 16. The apparatus ofclaim 15 wherein at least one of the pole pieces comprises aferromagnetic pole piece.
 17. The apparatus of claim 15, wherein themarine seismic source is configured to produce seismic energy within afrequency range of 2 Hz to 20 Hz at a water depth in a range of 15 to190 meters.
 18. The apparatus of claim 17, wherein the seismic energy iswithin a frequency range of 2 Hz to 10 Hz at a water depth in a range of35 to 190 meters.
 19. The apparatus of claim 15, wherein a combined massof the first plate and the first pole piece is within 5% of a combinedmass of the second plate and the second pole piece.
 20. The apparatus ofclaim 15, wherein, in the absence of the electric current, a stiffnessof the first and second plates provides a selected separation in therange of 0.5 to 5 mm between the first and second pole pieces.
 21. Theapparatus of claim 15, wherein the first plate is circular, and thesecond plate is circular.
 22. The apparatus of claim 15, furthercomprising a means for providing a repulsive force between the first andsecond plates.
 23. The apparatus of claim 22, wherein the interiorcavity is pressurized at a selected pressure in the range of 1-12 Bar,and wherein the selected pressure is operable to provide the repulsiveforce between the first and second plates.
 24. The apparatus of claim22, further comprising a plurality of permanent magnets operable toprovide the repulsive force between the first and second plates.
 25. Theapparatus of claim 22, further comprising at least one gas pistondisposed between the first and second pole pieces, wherein the at leastone gas piston is configured to provide the repulsive force between thefirst and second plates.
 26. A method of manufacturing a geophysicaldata product, the method comprising: obtaining geophysical data byactuating a marine seismic source to produce seismic energy, theactuating by: passing electric current through a wire coil disposed in afirst groove defined in a first pole piece, the first pole piece coupledto a first plate that defines a first central axis and a first outeredge; attracting a second pole piece toward the first pole piece, thesecond pole piece coupled to a second plate that defines a secondcentral axis and a second outer edge, the first central axis coaxialwith the second central axis, and the first plate parallel to the secondplate; bending the first and second plates toward each other along thefirst and second central axis; and maintaining separation of the outeredges of the first and second plates by way of a hoop disposed betweenthe first and second plates at the outer edges; and recording thegeophysical data on a tangible computer-readable medium.